Laser diodes emit light when current is passed through the diode. The optical power of the light varies as the drive current through the diode is varied. FIG. 1 shows a prior art laser diode drive circuit that includes a laser diode 110, a digital-to-analog converter (DAC) and driver circuit 108, and an inverse laser model 101. In operation, a value representing a desired optical light power is provided to the inverse laser model 101. Inverse laser model 101 maps the desired optical power value to a different drive value to compensate for the nonlinear operating characteristic of diode 110. The drive value is then provided to the DAC/driver circuit 108, which then drives diode 110 with a drive current, resulting in emitted light 112.
FIG. 2 shows prior art curves used in the drive circuit of FIG. 1. Curve 220 shows an idealized nonlinear diode operating characteristic. This typical diode curve is well known and shows that a negligible (or zero) amount of light is produced below a certain threshold current, and then above the threshold, the amount of light produced is a nonlinear function of drive current.
Curve 230 shows a desired linear relationship between desired optical power and output optical power that is actually produced by the diode. Curve 210 shows the inverse laser model that, when combined with the operating characteristic shown by curve 220, will result in the desired linear relationship shown at 230.
Using an inverse laser model as shown in FIGS. 1 and 2 works well to linearize the relationship between desired optical power and output optical power when the laser diode operating characteristic is time-invariant. Unfortunately, laser diode characteristics are not always time-invariant. For example, the output optical power of the laser diode may vary with age, temperature changes, and other factors.